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IRON

169

ferro-tungsten, etc., are not completely attacked by copper-potassium chloride,
and cannot, therefore, be analysed in this way (see p. 168).

2. Determination of the Graphitic Carbon (graphitic carbon and
annealing carbon).—These are not altered by hot dilute nitric acid, whilst
the combined carbon (hardening carbon and carbide carbon) are converted
into volatile or soluble products. On this difference is based the method
of separation of the two types, of carbon.

As a rule, i gram of grey cast-iron or 2-3 grams of white cast-iron or
5-10 grams of steel, are treated in a tall, narrow beaker covered with a
watch glass with nitric acid of D 1-2 (about 25 c.c. per gram of metal), the
beaker being cooled at first to prevent an excessively violent reaction.
When the reaction begins to slacken, the beaker is heated on a sand-bath
to complete the action and, if much silicon is present, when the metal
is entirely attacked 0-5-1 c.c. of pure hydrofluoric acid is added from a
platinum crucible without touching the sides of the beaker.

The liquid is gently boiled for i-i|- hour and is then diluted with water,
the insoluble matter being allowed to settle and collected on a Gooch crucible
and washed with hot water until free from acid.

After being dried at not too high a temperature, the asbestos and graphitic
residue are oxidised either in the Corleis apparatus with chromic and sul-
phuric acids (see p. 164) or in an open tube in a current of oxygen (see p. 167).

According to Ledebur, the graphitic carbon separated may be collected and
weighed directly on a filter dried at 100° and tared. To ensure complete removal
of extraneous substances, the residue should be washed twice with hot water,
twice with hot 5 % caustic potash solution, twice with hot water, twice with hot,
dilute hydrochloric acid (i : 3), and finally three times with hot water. In this
case the addition of hydrofluoric acid to eHminate every trace of silicon is, of
course, indispensable.

3. Determination of the Combined Carbon (carbide carbon and
hardening carbon).—The combined carbon in cast-iron may be calculated
as the difference between the total carbon and the graphitic carbon deter-
mined by the methods already given.

In malleable iron and especially in steel, which do not contain graphitic
carbon, the determination of the combined carbon (in this case, the total)
may be carried out by the rapid colorimetric method.

COLORIMETRIC   DETERMINATION   OF   THE   COMBINED   CARBON   (Eggertz

method). This is based on the fact that hot nitric acid leaves the graphitic
carbon unaltered, while the combined carbon is partly liberated as gas
(hardening carbon) and partly dissolved in the acid (carbide carbon), which
is coloured a more or less intense brownish-red in dependence on the quantity
of carbon.

' In general, the proportions of carbide carbon and hardening carbon in
untempered steels 1 are in almost constant ratio, so that, if parallel tests
are made on the sample under examination and on a steel with a known
content of carbon, the solutions obtained will have colour intensities pro-
portional, not only to the content of carbide carbon but to the content of
total combined carbon.

1 This method is consequently inapplicable to tempered steels.mulhod